This invention relates generally to optics, optical waveguides, and optical systems and devices. It is especially relevant to LIDAR (Light Detection And Ranging) and optical communication systems where optimal performance at a short range is desired. In its simplest form, a LIDAR system has an optical transmitter (typically a laser) and an optical receiver placed side-by-side, pointed in the same direction. Collectively these are known as a transceiver. The transmitter provides a short-pulsed beam of light that is directed at a target. Direct detection of the intensity of light returned from the target can be used to determine the range to the target Coherent detection of the light returned from a moving target enables its Doppler frequency to be measured, and thus its velocity. Velocity determination can also be done with a continuous wave (CW) laser.
There are several common transceiver designs, and one of the simplest is depicted in FIG. 1. In this configuration a laser 2 transmits a beam of illumination 4 toward a distant target (not illustrated). A receiver telescope 6 includes an objective 8 that focuses light in its field of view (FOV) 10 returned from the target, as by reflection or backscattering, onto a detector 12. This configuration is often referred to as a bi-axial, dual-aperture transceiver because the optical axis of the beam 4 from the laser 2 and the axis of the FOV of the receiver telescope 6 are independent, and because the laser beam and detector do not share the same aperture. This geometry is problematic because it creates a ‘blind spot’ 14 in front of the receiver in the region where the transmitter beam and receiver FOV do not overlap. The length of the blind spot represents the minimum range of operation, and it will be appreciated that if the axes of the illumination beam 4 and the FOV of the telescope 6 are inclined toward each other to reduce the minimum range by shortening the blind spot 14, a new blind spot (not illustrated) will then be created beginning at the distant point where the laser beam and the FOV no longer overlap because of their divergence.
FIG. 2 illustrates an improvement on the design of FIG. 1. This is commonly referred to as a co-axial, dual-aperture design. In this configuration, the illuminating beam from the laser 2 is directed by mirrors 16 to be co-axial with the FOV or the returned light 10. This configuration still has a blind spot because the central mirror obscures a portion of the telescope FOV, but it is much smaller than the blind spot of the bi-axial, dual-aperture transceiver illustrated in FIG. 1. Moreover, the laser beam 4 and receiver FOV 10 can be made to overlap all the way out to infinity, such that there is no distant blind spot.
In some applications, however, it is highly desirable to have no blind spots. This can only be accomplished by combining the laser beam into the FOV of the receiver to completely overlap the two fields at any range. This requires a coaxial, single-aperture transceiver design.
The configuration illustrated in FIG. 3 uses the properties of highly polarized light from the laser 2 to fold it into the FOV of the telescope by using a polarizing beam splitter (PBS) 18 in front of the detector. There is a penalty for the arrangement of FIG. 3, however, because, in general, light from non-metallic targets is randomly polarized, so only 50% of the return signal passes through the PBS 18 on its way to the detector. This is worse when the LIDAR is used on partially polarizing targets or even more so in the case of purely specular targets. In the latter case, all of the return signal would be directed back to the laser unless one uses the usual trick of placing a quarter-wave plate between the PBS 18 and the entrance to the telescope with its fast axis at 45°.
There are ways to get around the return power loss penalty of FIG. 3, most of them involving combinations of PBS's in other forms, Faraday effect materials, and so called ‘walk-off’ crystals made from birefringent materials. One commercially available device that could be used is called an optical circulator, which is a compact, three-port device that allows light to travel from ports 1 to 2, but light traveling in the reverse direction, from port 2 to 1 is redirected to port 3.
FIG. 4 illustrates a coaxial, single-aperture LIDAR transceiver with a fiber optic circulator 20. The circulator directs light from the laser 2 to an optical fiber 22 having its exit end positioned at the focal point of the objective 8. Light returning from the target is focused onto the end of the fiber 22, and the circulator directs this returning light to an output fiber connected to detector 12. This system offers the advantage that the transceiver can be located remotely from the laser 2 and detector 12, which is useful in those situations where space is at a premium (e.g., an aircraft). Moreover, there is only one bulk optic element (the objective lens in this case), which provides advantages in both size and weight over two-aperture transceivers.
There is a subtle problem with the transceivers illustrated in FIGS. 3 and 4. The main characteristic of these transceivers is that light from the transmitter shares optical elements with the receiver. Unfortunately, real-world optical elements do not behave ideally, and some of the illuminating light from the laser 2 necessarily makes its way to the receiver.
As an example, a polarizing beam splitter (PBS) generally leaks at least some of the polarized light arising from reflections at the interfaces. Additionally, imperfections on the reflecting surfaces scatter the impinging light in all directions. With reference to FIG. 5, a polarizig beam splitter 18 is shown with illuminating beam 4′ incident thereon. A polarizing reflecting surface 24 reflects most of the light 4′ to form the outgoing beam 4, but some of the beam 4 is reflected at planar surface 26 of the prism forming the polarizing beam splitter. Light reflected from surface 26 can pass back through the polarizing reflecting surface to as beam, which will be incident on the detector. Also, some of the light 4′ will be transmitted by the surface 24 as shown at 4″, and that light will be scattered by imperfections in the reflecting surface to form scattered light 30.
Consider first the problem of leakage. Some of the incoming light will be partially transmitted through the reflecting surface 24 of the PBS (typically 0.01 to 0.1%) to form beam 4″. The ‘leaked’ light from the transmitter direction does not pose a problem as long as adequate steps are taken to shield the detector from it, but this can be difficult at high pulse powers. Next, the back-reflection 28 from the output surface 26 can be made quite low with the appropriate anti-reflection (AR) coating (say 0.1%) and since the back reflected light has the same polarization as the incident light, most of it will be directed back towards the transmitter. But enough of it will reach the detector with an intensity that is comparable to or greater than that of the return signal from the target.
Another imperfection is scattering. PBS's are made from two right prisms that have their hypotenuses coated with a special dielectric layer and then bonded together. There will be some scatter from this interface (24 in FIG. 5) due to microscopic polishing and coating imperfections. Although the intensity of the scattered light 30 can be made extremely low by careful manufacturing (usually at significantly increased cost), the amount of scattered light that reaches the detector from the diagonal surface can be comparable to the signal returned from the target. In general, all interfaces generate some degree of scattering, but the diagonal surface is in the FOV of the detector, so it contributes the most.
There are other sources of scattering that arise from the bulk properties of the optic elements themselves (e.g., Rayleigh and Brillouin scattering). The contributions from these sources are extremely small compared to ‘interface’ sources, but they are fundamental properties of bulk matter. These bulk properties impose ultimate limits on the weakest signal that can be detected when the transmitter and receiver share the same optical elements.
Since fiber optic components like circulators are composed of many small optical elements similar to PBS's they suffer from the same problems. In fact the best commercially available circulators presently have an isolation between the transmitter port and the receiver port of about 60 dB. While this may be acceptable for telecommunication systems, it presents a problem in LIDAR systems, as return beams are usually reduced by more than 60 dB relative to the transmitter.
Thus, scattering, back-reflection, and light leakage from the optical elements shared by both the laser and detector in a coaxial, single-aperture transceiver generate light at the detector that is usually comparable to that of the weak signal returned from the target. Special care and attention must be paid to the design and construction of these transceivers in order to isolate the high-power, out-going laser beam from the return signal.
In range finding applications, the isolation may be so inadequate and the pulse peak power so high that the light from the transceiver alone temporarily blinds sensitive detectors. In this situation one can temporarily power off the receiver (gating), but this creates a blind spot at close range due to the switching times involved (or detector recovery time if it is not switched off). The inability of a LIDAR system to determine distance at close range because of isolation is sometime refered to as the “t=0 problem”.
For Doppler CW LIDAR systems, insufficient isolation causes a permanent Doppler signal at zero velocity (also called the “v=0 problem”). This signal will generally have a linewidth equal to that of the transmitter laser, and thus limit the minimum speed that can be detected. Pulsing is required to eliminate the zero-velocity Doppler signal to allow measurement of very low speeds, but this is ineffective if one desires to do velocity sensing and very close ranges.
One approach to get around the inadequate isolation offered by the circulator method has been described in U.S. Pat. No. 6,757,467 (Rogers). In this approach, a double clad fiber having a single mode core has its tip placed at the focal point of a lens so that transmitter light leaving the single mode core is substantially collimated by the lens. Return light from a hard target is then collected by the lens and focused back on the single mode core. The image of this returned light is substantially larger than the single mode core, so most of the return light is collected by the inner cladding of the fiber, extracted, and then transferred to a detector to make a useful LIDAR transceiver.
However this approach also suffers from an isolation comparable to the circulator approach. This is because light propagating in the single mode core actually extends beyond the core (so-called evanescent wave). When this light reaches the tip of the fiber, some of the evanescent wave is reflected back into the inner cladding. The intensity of this back-reflection is usually stronger than the return. Another drawback to this method is that light in the inner cladding is multimode, making coherent detection for velocity sensing poor so the approach described by U.S. Pat. No. 6,757,467 is mostly relegated to range finding.
Another approach that circumvents the use of cirulators involves placing fiber optic wave guides side-by-side as close as possible. One fiber is the transmitter while the other fiber is the receiver. The transmitter tip is place at the focal point of an objective lens to create a substantially collimated beam. The transmitter beam creates a bright spot on the target, and the objective lens then creates an image of this spot centered on the transmitter fiber. By placing the receiver fiber within this image, a small amount of signal can be extracted for range finding and velocity sensing purposes.
Upon analysis it is clear that this approach has great merit because it is capable of a high degree of isolation. However it is also clear that the maximum sensing range of this approach is limited by the space between the receiver and transmitter fibers. Those previous groups used commercially available telecom fiber (125 μm in diameter) placed in V-grooves and did not appear to make an effort to bring the waveguides closer together to improve the range of their transceivers.
The aim of the invention described herein improves upon the last approach by using several different methods to take commercially available optical fiber and bring them closer together to extend the range of this type of optical transceiver, while preserving the excellent isolation offered by the design,
It finds it usefulness in LIDAR systems that are required to sense range and velocity (among other possibilities) from a range of zero to several hundred meters. One such use for such a LIDAR system is to assist in the landing of manned and unmanned aircraft, or for collision avoidance in autonomous vehicles.